Studies will be continued of the aromatase enzyme complex of human placental microsome which catalyzes the formation of estrogens from androgens. The individual components have been separated into soluble fractions containing NADPH cytochrome c reductase and cytochrome P-560 which will be purified and further characterized. Structure-activity relationships will be further studied using steroid analogues. Possible control mechanisms involving non aromatizable steroids operative in the placenta, ovary and peripheral tissues will be investigated. The effects of placental estrogens in fetal tissues in culture are being investigated.